1. Field of the Invention
This invention pertains to the field of ultrasonic welding of plastics generally and specifically to a process for controlling compression of a seal or spring during the welding process.
2. Description of the Related Art
U.S. Pat. No. 4,631,685 to Peter, incorporated herein by reference, illustrates the prior art which applicants are aware of. Known processes for ultrasonic welding operate from dimensional stack up. Each component is dimensionally measured at the point of weld energy application and summed to other measurements to calculate a total stack up dimension of components at the intended weld site. Once the total stack up distance is calculated, a particular weld distance which has been determined through trial and error is summed with the stack up distance. This gives a calculated weld termination point.
In operation, the components are stacked together and positioned in line with the ultrasonic welding horn. The horn is caused to move towards the components with ultrasonic energy being supplied to the horn. The position of the horn is monitored by a linear position encoder. The position is compared to the earlier calculated weld termination point. Once the horn has reached the termination point, ultrasonic energy is ceased to be applied to the horn and the horn is briefly maintained in position. The weld junction is allowed to set and the horn then removed in preparation for the next weld cycle. Further details of the operation may be gleaned from the Peter disclosure.
While this welding process performs satisfactorily for certain types of operations, applicants have had the task of very reliably welding together two pieces of plastic which in turn apply a compressive force at a location remote from the weld to an elastomeric gasket. While stack up calculations provide some repeatability, unfortunately several variables are introduced which can not be determined based upon stack up measurements alone.
From lot to lot and even within lots, elastomeric gaskets vary in distance of compression required to produce a given compressive force. The reliability of a seal is dependent upon the compressive force applied to the seal, not the distance of compression. For example, if too great a compressive force is applied to the seal, relative motion between the parts being sealed results in greatly accelerated wear to the gasket. Where too light a force is applied, certain changes such as temperature variations which affect each of the components differently may result in a reduction of sealing force below that which would be required for liquid or vapor permeation of the seal.
Additionally, and similar to the elastomeric gaskets, from lot to lot and within lots the parts which are to be welded together have varying shapes and dimensions. The variations in shape are a direct consequence of the plastic molding operation used to form the parts. Some parts may exhibit much greater curvature from edge to center than others and there will likely be variances in thickness at particular points along the plastics depending upon shrinkage variations within the plastic during cooling after molding.
A measurement of stack up at the weld point does not identify dimensional variations which must be compensated for in order to provide a predictable sealing compression. Further, strictly dimensional measurements do not anticipate the variation of thickness between the weld location and the location of gasket compression which will result in different forces being transmitted to the gasket location from the weld site for a given identical horn displacement.